Implement module instance context APIs (#2436)

Introduce module instance context APIs which can set one or more contexts created
by the embedder for a wasm module instance:
```C
    wasm_runtime_create_context_key
    wasm_runtime_destroy_context_key
    wasm_runtime_set_context
    wasm_runtime_set_context_spread
    wasm_runtime_get_context
```

And make libc-wasi use it and set wasi context as the first context bound to the wasm
module instance.

Also add samples.

Refer to https://github.com/bytecodealliance/wasm-micro-runtime/issues/2460.

core/config.h

@@ -480,4 +480,9 @@
 #define WASM_MEM_DUAL_BUS_MIRROR 0
 #endif
 
+/* The max number of module instance contexts. */
+#ifndef WASM_MAX_INSTANCE_CONTEXTS
+#define WASM_MAX_INSTANCE_CONTEXTS 8
+#endif
+
 #endif /* end of _CONFIG_H_ */